LeetCode 83. Remove Duplicates from Sorted List

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->2->3.

给定一个已排序的链表，删除其中重复的结点。
思路：

class Solution {
public:

    ListNode* deleteDuplicates(ListNode* head) {
        if (head == NULL) return head;

        ListNode **pp = &head;
        int preValue = (*pp)->val;
        pp = &((*pp)->next);
        while (*pp)
        {
            int value = (*pp)->val;
            if (preValue == value)
            {
                ListNode *p = *pp;
                *pp = (*pp)->next;
                delete p;
            }
            else {
                preValue = value;
                pp = &((*pp)->next);
            }
        }
        return head;
    }
};